The present invention relates to the art of induction heating and, in particular, to a method and apparatus for the heat treating of camshafts for internal combustion engines.
The invention will be described with reference to engine camshafts. however it will be appreciated that the invention has broader aspects and may, for instance, be used for various elongated workpieces having spaced hardened surfaces which must be individually heated without affecting the hardened integrity of an adjacent previously hardened surface.
Induction hardening is a proven process for hardening the cam lobes for the camshafts of internal combustion engines. In one system, individual camshaft lobes are induction heated, one at a time, with relatively low power densities to the elevated hardening temperature. After heating, the camshafts are immersed into a quenching bath. This sequential method is time consuming and costly. Other methods have been developed for heating multiple cam lobes at a time ultimately leading to the simultaneous heating of all the cam lobes followed by immersion of the entire camshaft in the quenching bath. Because of the number of inductor coils used for simultaneous heating, power supply limitations restrict this approach to low power density systems, which provide a substantial hardening depth but not a consistently uniformly hardened surface.
Recently, roller lifters have been adopted to provide greater service life and accuracy in the actuation of the engine valve train. These rollers impose substantially higher compressive loads on the cam lobe. Accordingly, the uniformity of hardening is of utmost importance to resist lobe deformation and wear. This has lead to the development of high power density, short time induction heating of the cam lobes. Because of the higher power requirements, such methods are restricted to heating one cam lobe at a time. Generally, this has involved placing the camshaft in a vertical orientation and each cam lobe is heated and quenched sequentially until all the cam lobes are hardened.
The high power density induction heating of camshafts presents certain problems in attaining an overall uniformity of hardness. Inasmuch as the cam lobes are closely spaced, the peripheral edges of adjacent camshaft lobes experience stray induction heating. Previously hardened cam lobes are thus prone to tempering, leading to an undesirable decrease in hardness and uniformity. While flux shields have been used in other applications for limiting the effects of stray induction heating, their use in conjunction with the extremely closely spaced cam lobes adversely affects the flux field of the cam lobe being heated. Accordingly, there is a need for high power density induction heating systems for camshafts that will insure the efficient production of uniformly hardened cam lobes.